Corrosion resistant electrodes for electrophoretic mobility measurements and method for their fabrication

ABSTRACT

An electrode for use in instruments capable of measuring the electrophoretic mobility of particles in solution is disclosed. The electrode is comprised of an inexpensive support member, generally made of titanium, onto a flat surface of which has been connected, generally by microwelding, a flat electrically conductive but chemically inert foil member, preferably platinum. A uniform texture can be generated on the exposed surfaces of the electrode by various means including tumbling the electrode with an abrasive. An oxide layer can be generated on the support member by soaking the composite electrode in an appropriate medium, protecting the exposed surface of the support member from fluid contact with the sample solution, while the foil member, unaffected by the oxidation process, is able to contact the sample solution.

RELATED APPLICATIONS AND PATENTS

The following patent applications relate to measurement of theelectrophoretic mobility of particles and are hereby incorporated byreference:

U.S. Pat. No. 8,441,638 B2, H.-T. Hsieh and S. Trainoff, “Apparatus tomeasure particle mobility in solution with scattered and unscatteredlight,” issued May 14, 2013.

U.S. Pat. No. 8,525,991 B2, H.-T. Hsieh and S. Trainoff, “Method tomeasure particle mobility in solution with scattered and unscatteredlight,” issued Sep. 3, 2013.

P.C.T. Application PCT/US12/49641, S. Trainoff, “Method and apparatus tosuppress bubbles in optical measurement cells,” filed Aug. 5, 2012, andclaiming priority to U.S. Provisional Application 61/515,796, filed Aug.5, 2011.

BACKGROUND

The invention discloses an innovative electrode design generally for usein instruments designed to measure the electrophoretic mobility ofmacromolecules in solution, wherein the motion of charged particles in asolution, subject to an applied electric field, is measured. Althoughthe present disclosure will refer to macromolecules throughout much ofits description, measurements using the inventive apparatus disclosedherein may include more generally, all classes of small particlesincluding emulsions, viruses, nanoparticles, liposomes, proteins,macro-ions, and any other solution constituents whose size may liebetween about a half and a few thousand nanometers. Thus whenever a termsuch as “molecule,” “macromolecule,” “particle,” or “macro-ion” is used,it should be understood to include all of the aforementionedsolution-borne objects to be subject to some form of opticalmeasurement.

Electrophoretic mobility is the directly measurable and most widely usedquantity to characterize the charge of molecules, or other particles insolution. Once measured, the electrophoretic mobility can be used inturn to determine the effective charge, Ze, carried by such molecules aswell as their so-called zeta potential. The interface between the groupof ions tightly bound to the particle and those of the surroundingsolution that do not move with the particle defines the hydrodynamicshear plane. The zeta potential represents the electrostatic potentialexisting at this shear plane. It is an objective of the presentinvention to improve the reliability of measurements of electrophoreticmobility, effective charge, and zeta potential of molecules andparticles in solution contained within a measurement cell, as well as toimprove the durability of the instruments and their components whichpreform these measurements.

Several techniques have been developed and are available for measuringmobilities including light scattering methods such as heterodyne DLSincluding both laser Doppler electrophoresis, LDE, and phase analysislight scattering, PALS. These techniques involve measuring lightscattered from moving particles, whereby such scattered light carriesinformation relating to such motion and from which the associatedelectrophoretic mobility of the particles may be determined.

Instruments that measure electrophoretic mobility must, by necessity,apply an electrical field, generally between two electrodes, in a fluidsample to induce electrophoresis. The resulting motion is generallyprobed optically to determine the resulting sample velocity. Thiscompromises a first principles measurement of mobility, which is wellestablished as an important parameter for predicting the stability ofcolloidal suspensions. In recent years electrophoretic mobility isfinding new use in determining the stability of molecular solutions.Over the years, many electrode designs have been used. An objective ofthe present invention is to provide an inexpensive electrode thatapplies a uniform field and is mechanically and chemically durable.

A BRIEF DESCRIPTION OF THE INVENTION

An innovative electrode design is described, which, in its variousembodiments, offers the advantages of a planar surface in contact withthe sample fluid which is less prone corrosion than electrodes createdby standard electroplating techniques. In one embodiment this isachieved by microwelding a platinum foil surface to a support made of aless expensive material such as titanium. More specialized embodimentsaid in the prevention of bubble formation on the electrode surfaces andlimit the surfaces of the electrode in contact with the fluid to theplanar electrode surface.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the front and side view of an exemplar optical cell formeasurement of electrophoretic mobility.

FIG. 2 illustrates the elements of an embodiment of the invention with aplatinum foil member to be microwelded onto a titanium support member.

A DETAILED DESCRIPTION OF THE INVENTION

As discussed above, electrophoretic mobility measurement instrumentsgenerally employ two electrodes comprising planar surfaces placedparallel to each other, between which is placed a liquid sample andacross which an electric field is generated. The applied electric fieldinduces electrophoresis within the fluid sample. A graphicrepresentation of the elements of an optical electrophoretic mobilitymeasurement cell 100 is shown in FIG. 1. A beam of light 101, typicallygenerated by a laser, passes through an optically transparent window 102and into a sample chamber 103, wherein a fluid sample has been placed,generally through injection ports within the top plate 105. The beamleaves the chamber through exit window 104, and the physical propertiesof the sample molecules are derived based on measurements of theemerging beam and light scattered therefrom. The necessary electricfield is generated between electrodes 106. It should be noted that manyvariations on this optical measurement cell exist, and the utility ofthe present invention is not limited to use with only a flow throughsystem such as that shown in FIG. 1. One example of a measurementchamber for which the present invention may be advantageous is a samplecuvette containing the necessary elements of the measurement cell, whichmay be filled with sample outside of the measurement instrument andsubsequently placed within the path of the beam 101 within themeasurement instrument. While the particular elements of the instrumentand measurement cells with which the present invention may be utilizedmay vary for different instrument/cell designs, this disclosure isconcerned primarily with the electrodes used to generate the electricfield within these cells.

For many years, platinum has been the preferred electrode materialbecause it is chemically inert, even in high salt buffers, and does notoxidize. Pure platinum however, is very expensive so there is a strongincentive to minimize the amount of material that is used. A commonstrategy is to fabricate the electrodes out of an inexpensive material,such as stainless steel, and then protect them with an electroplatedplatinum coating. This technique is effective, but suffers from a numberof problems. In order to get good adhesion, the substrate must beexceptionally clean. Even a small amount of contamination on the surfaceor in the plating baths can give rise to coatings that crack or flakeoff. Even when extreme caution is taken to insure good adhesion, theresulting platinum coating is brittle. If the coating is made more thanabout 2-5 μm thick, the mechanical strain that develops during thecoating process frequently causes the film to crack, exposing theunderlying substrate. If the coating is made less than 2 μm thick, itmay be porous, again exposing the substrate. When an electrical field isapplied, the underlying substrate can corrode causing the plated surfaceto loosen. Further, when the electrodes are cleaned, either chemically,or by gentle mechanical abrasion, the coating can detach from thesubstrate surface. It is therefore of critical importance that platedsurfaces, with their attendant chemical and mechanical problems, beavoided in the fabrication of electrodes.

In one embodiment of the invention whose elements are shown in FIG. 2,the electrode 200 is comprised of a support member 201 made of anappropriate, but relatively inexpensive material, such as titanium ontowhich is welded a disc of bulk conductive foil 202. This microweldingaround the perimeter of the platinum foil may be performed by anelectron beam. The use of electron beams to microweld surfaces is wellknown in the art. An o-ring groove 203 and other physical elements maybe present as part of the support member. While platinum is a preferredmaterial for this foil surface, the invention should not be limitedthereto. Other chemically inert but electrically conductive materialssuch as gold or various alloys capable of being microwelded to a supportmember may also be used. As regards to platinum, or other expensivematerials, it is clear that the cost of a thin foil microwelded to aninexpensive support member is far less than that of an electrodecomposed of solid platinum. The inventive design thus offers aconsiderable cost savings while still providing the benefits of aplatinum electrode.

The foil surface itself can be made as thick as desired to insuremechanical robustness, while still minimizing the amount of materialconsumed. In a preferred embodiment, the platinum foil is roughly 100 μmthick insuring that it can be cleaned with mild abrasives withoutdamage.

One limitation of using bulk foils is that they are generally formed byextrusion through rollers giving rise to a smooth surface with smallscratches. These scratches can act as nucleation sites for electrolysisbubbles that are often formed during the measurement of samples in highconductivity solutions. To minimize the formation of bubbles, a uniformsurface texture is preferred. This limitation can be overcome withanother embodiment of the invention, wherein the electrode is etchedafter welding. Several methods can be used to achieve the desiredsurface etching, for example, the application hydrofluoric acid or ionbeam etching. In a preferred embodiment, the etching is achieved bymechanical tumbling with an abrasive. An advantage of mechanicaltumbling is that it is possible to adjust the tumbling time and abrasivebead size to easily control the final surface finish.

One limitation of some embodiments of the invention described above isthat both the platinum foil and titanium support member are in contactwith the fluid sample. In order to apply a uniform field, it isdesirable that only the parallel plates formed by the platinum foil arein contact with the fluid. This can be achieved with another embodimentof the invention wherein the composite electrode is soaked in a strongoxidizer that causes an oxide layer to grow on the exposed surfaces ofthe support member but wherein the platinum foil is unaffected. Apreferred method for achieving this oxidation layer on the supportmember is the soaking of the composite electrode in a solution of 30%hydrogen peroxide in water. The oxide layer may additionally begenerated with other oxidizers or electrochemically, by passing currentthrough the electrodes while bathed in a salt solution. The resultingelectrode is inexpensive, chemically inert, and mechanically robust.

As will be evident to those skilled in the arts of materials science,and optical and electrophoretic mobility measurements, there are manyobvious variations of the methods and devices of the invention andmethod for manufacture thereof that do not depart from the fundamentalelements that disclosed herein; all such variations are but obviousimplementations of the described invention and are included by referenceto our claims, which follow.

1. A corrosion resistant electrode for use with an electrophoreticmobility detector comprising a) an electrically conductive supportmember b) a chemically inert, electrically conductive foil memberconnected to said conductive support member.
 2. The electrode of claim 1wherein said electrically conductive support member is comprised oftitanium
 3. The electrode of claim 1 wherein said electricallyconductive foil member is comprised of platinum.
 4. The electrode ofclaim 3 wherein said platinum foil member is disc shaped.
 5. Theelectrode of claim 1 further comprising an o-ring groove.
 6. Theelectrode of claim 4 wherein said support member is welded to said foilmember about the perimeter of said foil member.
 7. The electrode ofclaim 4 where said platinum foil is 100±5 μm thick permitting the foilto be cleaned with mild abrasives without damage.
 8. The electrode ofclaim 1 wherein at least said electrically conductive foil member isetched to provide a relatively uniform surface texture.
 9. The electrodeof claim 8 wherein said etching is achieved by the application ofhydrofluoric acid.
 10. The electrode of claim 8 wherein said etching isachieved by ion beam etching applied to the exposed side of theelectrically conductive foil member.
 11. The electrode of claim 8wherein said etching is achieved by mechanically tumbling said electrodewith an abrasive.
 12. The electrode of claim 1 wherein said supportmember is comprised of a material whose surface, when exposed to astrong oxidizer causes an oxide layer to grow thereupon, and whereinsaid foil member is comprised of a material whose surface, when exposedto a strong oxidizer is resistant to an oxide layer forming thereupon.13. The electrode of claim 12 wherein an oxide layer is generated uponthe exposed surface of said support member by means of an oxidationagent.
 14. The electrode of claim 13 wherein an oxide layer is caused togrow upon the exposed surface of the support member by soaking saidelectrode in a solution comprised of hydrogen peroxide and water. 15.The electrode of claim 14 wherein said solution is composed of about 30%hydrogen peroxide and 70% water.
 13. The electrode of claim 12 whereinan oxide layer is generated upon the exposed surface of said supportmember by means of passing an electrical current through said electrodewhile it is bathed in a salt solution.
 16. A method for the fabricationof an electrode for use with an electrophoretic mobility detectorcomprising the steps of A. providing an electrically conductive supportmember; B. providing a chemically inert, electrically conductive foilmember; C. welding said foil member to said conductive support memberabout the perimeter of said foil member, forming, thereby a compositeelectrode; D. mechanically tumbling said composite electrode with anabrasive, providing thereby a uniform surface texture to the exposedsurfaces of each of said conductive support member and said foil member;and E. soaking said composite electrode in a solution comprised ofhydrogen peroxide and water, providing thereby an oxide layer on theconductive support member.